Matplotlib animate fill_between shape

Question

I am trying to animate a fill_between shape inside matplotlib and I don't know how to update the data of the PolyCollection. Take this simple example: I have two lines and I am always filling between them. Of course, the lines change and are animated.

Here is a dummy example:

import matplotlib.pyplot as plt

# Init plot:
f_dummy = plt.figure(num=None, figsize=(6, 6));
axes_dummy = f_dummy.add_subplot(111);

# Plotting:
line1, = axes_dummy.plot(X, line1_data, color = 'k', linestyle = '--', linewidth=2.0, animated=True);
line2, = axes_dummy.plot(X, line2_data, color = 'Grey', linestyle = '--', linewidth=2.0, animated=True);
fill_lines = axes_dummy.fill_between(X, line1_data, line2_data, color = '0.2', alpha = 0.5, animated=True);

f_dummy.show();
f_dummy.canvas.draw();
dummy_background = f_dummy.canvas.copy_from_bbox(axes_dummy.bbox);

# [...]    

# Update plot data:
def update_data():
   line1_data = # Do something with data
   line2_data = # Do something with data
   f_dummy.canvas.restore_region( dummy_background );
   line1.set_ydata(line1_data);
   line2.set_ydata(line2_data);

   # Update fill data too

   axes_dummy.draw_artist(line1);
   axes_dummy.draw_artist(line2);

   # Draw fill too

   f_dummy.canvas.blit( axes_dummy.bbox );

The question is how to update the fill_between Poly data based on line1_data and line2_data each time update_data() is called and draw them before blit ("# Update fill data too" & "# Draw fill too"). I tried fill_lines.set_verts() without success and could not find an example...

Thanks!

Answer 1

Ok, as someone pointed out, we are dealing with a collection here, so we will have to delete and redraw. So somewhere in the update_data function, delete all collections associated with it:

axes_dummy.collections.clear()

and draw the new "fill_between" PolyCollection:

axes_dummy.fill_between(x, y-sigma, y+sigma, facecolor='yellow', alpha=0.5)

A similar trick is required to overlay an unfilled contour plot on top of a filled one, since an unfilled contour plot is a Collection as well (of lines I suppose?).